The present invention generally relates to keyboard assemblies and, more particularly, is concerned with a push-button type keyboard assembly having a plurality of individual switch modules removably installed at switch sites on a printed circuit board of the assembly. The modules contain resilient conductive elements and components that shield the switch sites from leakage of externally or internally originating electromagnetic interference (EMI) and radio frequency interference (RFI) past the conductive elements and into electrical circuit paths on the board that contain the switch sites. In the case of internally originating interference, the switch modules retain the interference behind the keyboard assembly.
It is conventional to provide keyboard assemblies which include switches installed on a printed circuit board and which are composed of dome-shaped resilient snap-action conductive elements that interact with contacts at the switch sites in electrical circuit paths on the board. Typically, a frame having a plurality of openings which receive push-buttons overlies the circuit board with the push-buttons in registry with the switch sites and in overlying contact with the dome-shaped conductive elements installed at the switch sites.
Application of pressure on a dome-shaped conductive element via one of the push-buttons by a user results in depression and snap action tripping of the element from a convex to concave configuration causing closing of the contacts at the switch site of the element. Then, when the pressure is removed by the user, the element springs back to its original convex configuration causing opening of the contacts. The spring return force is generated upon tripping which upon release of the depressing force causes the dome to return to its original shape to open the contacts. It is desirable to use a dome-shaped conductive switch element because when depressed and actuated it produces a tactile feedback or snap action upon being tripped that is felt by the user to indicate switch closure.
Representative of the prior art which use dome-shaped conductive switch elements are the keyboard assemblies disclosed in U.S. Pat. Nos. to Schadow et al. (4,046,982), Pounds et al., (4,195,210), Puccini (4,288,672), Pounds (4,365,120), Nishida (4,385,218), Bouvrande (4,439,646), Holland (4,456,800) and Mital (4,476,355). While many prior art keyboard assemblies would appear to operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, most seem to embody certain shortcomings which make them less than an optimum design. One shortcoming is that the useful life of keyboard assemblies frequently depends upon the expected life of their dome-shaped conductive elements. Many keyboard assemblies are constructed in such a way that a defective or failed dome-shaped element cannot be replaced necessitating the replacement of the entire assembly. Another shortcoming with most keyboard assemblies is that a change in the design of one part of the assembly necessitates redesign of the remainder of the parts thereof. Such redesign is extremely costly. Still another shortcoming with most prior art keyboard assemblies is the general lack of an effective way to shield electrical circuits behind the keyboard from the deleterious effects of externally-originating EMI and RFI by preventing leakage thereof past the push-buttons and dome-shaped conductive elements to the printed circuit board of the assembly or, in the case of internally originating EMI or RFI, to retain the EMI or RFI behind the keyboard so it does not escape. EMI, RFI-shielded keyboard assemblies are generally required in military applications.
Consequently, a need exists for a push-button keyboard assembly which employs means to effectively block leakage of EMI and RFI, both externally and internally originating. The keyboard assembly also should adopt a construction which allows replacement of failed components and provides standardization of components to allow their interchangeability and, without the need for complete redesign when one part of the assembly is redesigned.